1. Field of Invention
The present invention relates to a focus detection device and a distance measurement device which are mounted on optical equipment such as a camera. The present invention provides the ability to obtain high-contrast object images and to perform excellent focus detection and distance measurement at all times.
2. Related Background Art
The technology for optically measuring distances to objects which exist in a plurality of directions is described in, for example, U.S. Pat. Nos. 4,749,848 and 4,916,302. This technology can allow a photographer to obtain distance distribution information and defocus quantity distribution information for objects which exist in an objective field and to recognize the placement of the objects in the objective field based on these types of distribution information.
According to this approach, a camera having CCD imaging elements or the like is used to take an image of the objects and a pair of resulting images having a parallax with respect to each other is used to perform well-known correlation calculations on the parallax images to determine a defocus quantity. In addition, distances to the objects with respect to each calculation area can be determined based on the principle of triangulation. Thus, these calculations can be performed equally on each area of the resulting images to obtain such distance and defocus quantity distribution information.
Alternatively, there has been conventionally known a focus detection device for a TTL camera. This is an automatic focus detection device which uses a so-called pupil splitting method to detect focusing conditions of an image-taking optical system from a relative deviation quantity of a plurality of object images generated by luminous fluxes from regions of each having a different pupil position in the image taking optical system. For example, this kind of autofocus device has been disclosed, which consists of an array pair, that is, a lens array positioned in the proximity of a primary image plane and an optical receiving element array positioned directly behind the lens array. Another example of this kind of autofocus device has also been disclosed, which consists of a field lens positioned on a primary image plane, two re-imaging lenses for re-imaging an image formed on the primary image plane onto a secondary image plane, and two image sensor arrays positioned on the secondary image plane.
Now, a prior art device will be described below with reference to FIG. 6.
Reference numeral 601 denotes an image taking lens, 602 denotes as quick return mirror, 603 denotes a sub-mirror, 604 denotes a field lens, 605 denotes a diaphragm, 606 denotes a secondary imaging lens, 607 denotes an area sensor, 608 denotes a focus detection portion, 609 denotes a focus output portion, 610 denotes a focus lens, 611 denotes a pentaprism, 612 denotes an eyepiece, 613 denotes a photographer""s eye, 614 denotes a shutter curtain, and 615 denotes a film plane.
A part of the luminous flux passing through the image taking lens 601 is imaged on the area sensor 607 by the quick return mirror 602 and the sub-mirror 603 through the field lens 604, the diaphragm 605, and the secondary imaging lens 606. Here, the detailed configuration of the focus detection optical system will be described below in detail with reference to FIG. 7.
The luminous flux is guided respectively from different pupil positions of the image taking lens 601 onto two imaging screens 607a and 607b of the area sensor 607 to be reimaged under an imaging magnification determined by the field lens 604 and the secondary imaging lens 606. The area sensor 607 is positioned at a location optically equivalent to an image taking film plane with respect to the image taking lens 601 and imaging screens 607a and 607b have a field of view equal to a portion of an image taking screen or the image taking screen itself, respectively. The diaphragm 605 has a function of a diaphragm as well as that of an infrared cut filter, which removes undesired light.
The above-mentioned configuration can allow the imaging screens 607a and 607b to have a parallax of different pupil positions of a predetermined image taking lens. The imaging screens with this parallax are used to perform well-known correlation calculations on signals in opposed blocks in the focus detection portion 608 of FIG. 6 to determine a distance to an object in a previous block as well as a defocus quantity. For this purpose, it is preferable that a high-contrast image is used because the correlation calculations are performed to detect the quantity of displacement of the image. The correlation calculations cannot be performed correctly on a low-contrast image, for which the focus detection cannot be performed. This determination is performed on a predetermined block to obtain distance information or defocus quantity information and the result is provided by the focus output portion 609. Based on the provided result, the image taking lens 601 is driven to achieve a correct focal position for implementing autofocus.
The optical axis of the remainder of the luminous flux passing through the image taking lens 601 is bent by the quick return mirror 602 and the remainder of the luminous flux is guided through the focus lens 610, the pentaprism 611, and the eyepiece 612 to the photographer""s eye 613 to be finally recognized as a field-of-view of an image for image taking.
Thereafter, by pressing a shutter release button, the quick return mirror 602, the submirror 603, and the shutter 614 are withdrawn to expose the film 615 (for taking the image).
The area sensor for focus detection usually has a sensitivity characteristic that it shows sensitivity over the entire visible spectrum. Since the area sensor performs sensitivity identification rather than color identification, it can produce outputs of the same level from inputs of different colors if they have the same sensitivity. This operation will be described below with reference to FIGS. 8A to 8C.
FIG. 8A shows an example of an object for image taking and in the drawing, a first color is represented by oblique-line hatching and a second color is represented by horizontal-stripe hatching. In addition, it is assumed that the second color is located in the background for the first color and that the second color performs a higher-contrast image as compared with a stripe pattern of the first color.
FIG. 8B shows wavelength as an axis of abscissa and sensitivity as an axis of ordinate for modeling the sensitivity characteristic of the area sensor for focus detection, which shows that the area sensor has a sensitivity characteristic that it shows sensitivity over the entire visible spectrum.
FIG. 8C shows an image obtained by the area sensor for focus detection when the first and second colors shown in FIG. 8A have the wavelength and sensitivity as shown in FIG. 8B, respectively. A graph in the lower portion of FIG. 8C shows an output level of the output image described above in one-dimensional representation. This indicates that the first and second colors have different wavelengths and that the same output level is produced due to the sensor""s sensitivity characteristic.
There is still plenty of room for improvement in order to provide a focus detection device and a distance measurement device which provide the ability to obtain a high-contrast object image and to perform excellent focus detection or distance measurement operations at all times.
In one aspect, this invention has a plurality of light receiving sensors having different sensitivity characteristics, a focus detection circuit for performing focus detection based on the output of any one of the light receiving sensors, and a determination circuit for determining whether the focus detection result is appropriate, and if the determination circuit determines that the result is not appropriate, the focus detection circuit performs focus detection based on the output of another light receiving sensor. This provides the ability to perform focus detection from a high-contrast object image at all times, irrespective of the color of an object.
In another aspect, this invention has a plurality of light receiving sensors having different sensitivity characteristics, a distance measurement circuit for performing distance measurement based on the correlation among a plurality of pupil-split images obtained from the output of any one of the light receiving sensors or among a plurality of images with a parallax, and a determination circuit for determining whether the distance measurement result is appropriate, and if the determination circuit determines that the result is not appropriate, the distance measurement circuit performs distance measurement based on the output of another light receiving sensor. This provides the ability to perform distance measurement through correlation calculations of a high-contrast object image at all times, irrespective of the color of an object.